This invention relates to a solder connection composed of a lead-free tin-base alloy containing indium or bismuth. More particularly, this invention relates to a method for forming such solder connection having a liquids temperature that is increased relative to the initial solder alloy.
One method for forming a solder connection, for example, for physically and electrically connecting components of a microelectronic package, utilizes a solder paste. A typical paste is composed of a power formed of a solder alloy and dispersed in a liquid vehicle. One advantage of the paste is that it may be conveniently applied to a faying surface, for example, by screen printing. Following application, the paste is dried and heated to reflow the solder alloy to form a liquid phase that wets the faying surfaces. Wetting provides intimate contact between the molten alloy and the solid faying surface and is essential in obtaining a strong bond. A flux may be added to promote wetting by removing oxides or other contaminants from the faying surfaces during the early stages of heating. Nevertheless, reflow of the solder must necessarily extend for a sufficient time not only to melt the alloy, but also to permit fluxing and wetting to be completed.
In the past, most common solders were based upon tin-lead alloy. Tin and lead form a eutectic composition having a melting temperature of about 183.degree. C., which is suitably low to permit reflow while avoiding thermal damage to the components, but is sufficiently high to withstand temperature excursions of the type experienced by the package during normal operations. As used herein, melting temperature refers to the liquidus temperature, that is, the temperature above which the alloy exists as a uniform liquid phase, as opposed to the solidus temperature below which the alloy exists as a solid phase. It is desired to eliminate lead from solder connections. To this end, it has been proposed to utilize tin alloys containing bismuth or indium. Tin and bismuth form a eutectic composition containing about 60 weight percent bismuth and having a melting temperature of about 138.5.degree. C. Similarly, tin and indium form a eutectic composition containing about 50.9 weight percent tin and having a melting point of about 120.degree. C. Such low temperatures are not deemed adequate to withstand the thermal cycling experienced during typical usage. Thus, there is a need for a method for forming a solder connection that is based upon tin-indium alloy or tin-bismuth alloy and that has an increased melting temperature suitable for electronic applications.